A basic problem associated with any communication system is that of adding to the main signal stream a given number of supplementary signals which may also have a service function.
In the case of digital transmission systems, the addition of such supplementary signals gives rise to considerable technical problems and is generally very expensive.
The techniques used till now are mainly based on three types of solution:
(A) THE ADDITION OF SEPARATE SIGNAL LINKS; (B) FREQUENCY MULTIPLEXING OF THE ADDITIONAL SIGNALS WITH THE MAIN INFORMATION SIGNAL; AND (C) TIME MULTIPLEXING THE SEVERAL TYPES OF SIGNAL.
Even though the first solution results in a rather simple and reliable system from the technical standpoint, it entails a considerable increase in circuitry costs.
The solution offered by frequency multiplexing among supplementary signals and the main information signal necessarily imposes, as known to the skilled in the art, a restriction in the frequency spectrum assigned to the main signal stream and the use of expensive filters capable of preventing mutual interference between the signals of the main and supplementary streams; in any case this type of solution entails a degradation of the characteristics of the main information signal because of the filtering involved.
Finally, time multiplexing the main and additional signal streams requires an identification of the mainstream frame structure, the introduction of modifications, and the rearrangement of the signals into a new frame structure, which includes also the additional messages.
This type of operation is rather burdensome in terms of costs, in particular in cases where the frame of the main signal stream has been set up in a place different from that in which supplementary signals should be added or extracted.